Method of making a high voltage deposited fuel cell component

ABSTRACT

This invention pertains to a method of making an improved high voltage deposited fuel cell component. The improved fuel cell component is especially adaptable to high voltage, high power applications in that a large number of planar cell assemblies may be stacked in a compact manner to afford a relatively high power density.

United States Patent 1191 1111 3,784,415 Hopkins 1 1 Jan. 8, 1974 [5METHOD OF MAKING A HIGH VOLTAGE 3,440,105 4/1969 Yamamoto et :al.136/175 x DEPOSITED FUEL CELL COMPONENT Inventor: Ralph E. Hopkins,Falls Church, Va.

Assignee: The United States of America as represented by the Secretaryof the Army, Washington, DC.

Filed: Aug. 3, 1972 Appl. No.: 277,546

Related U.S. Application Data Division of Ser. No. 110,499, Jan. 28,1971, Pat. No. 3,717,506.

us. Cl. 136/175 Int. Cl.. H0lm 27/02 Field of Search 136/175 ReferencesCited UNITED STATES PATENTS 12/1965 Solomon et a1 136/175 X PrimaryExaminerA. B. Curtis Assistant Examiner-Thomas A. Waltz Attorney-EdwardJ. Kelly, Herbert Berl and Glenn S. Ovrevik [57] ABSTRACT 1 Claim, 9Drawing Figures PATENTEDJAH a 1974 SHEET 10F 3 OOO o M w r 1 METHOD OFMAKING A HIGH VOLTAGE DEPOSITED FUEL CELL COMPONENT DIVISIONALAPPLICATION This application is a division of US. Pat. No. 3,717,506 ofRalph E. Hopkins entitled High Voltage Deposited Fuel Cell, Ser. No.110,499, filed .Ian. 28, 197i.

GOVERNMENT USE The invention described herein may be manufactured andused by or for the Government of the United States of America forgovernmental purposes without the payment to me of any royaltiesthereon.

BACKGROUND OF THE INVENTION Fuel cells utilizing a thin layer of solidelectrolyte as the medium which functions to conduct mobile ions betweenplanar electrode surfaces are well known. Such electrolyte has therecognized advantage of a low internal resistance and thus does notintroduce significant ohmic losses with consequent internal heating problems. However, the open circuit voltage of fuelcells utilizing solidelectrolyte is relatively low, in the, order of 1.1 volts, anda numberof such fuel cells must be electrically connected inseries to provide ahigher voltage output. While this problem of serial electricalconnectionhas been solved in the case of large area liquid electrolytedevices, by stacking low voltage modules,

The discrete fuel cell elements are serially connectedanode to cathodeby a relatively complex concentric conductive ring structure. It will beappreciated that the bulky structure described above is difficult tomanufacture and inherently is not adaptable to a compact stackedassembly.

SUMMARY OF THE INVENTION The fuel cell device. provided by thisinvention con.- sists of. a battery offinite fuelcell elementselectrically connected in a series-parallel arrangement which affords ahigh. voltage, high current, lowinternalimpedance output. In accordancewiththeinvention, a plurality of finite fuel cell elements may be juxtadisposed in a common plane and a group of similar; common planestructures maybe stacked with appropriate physical separation betweencommon planes defining gas passageways. Each of the common planestructures is impervious to commonlyutilized gases, such as oxygen andhydrogen, yet affords significant ion mobility in the solid electrolytemedium. It is recognized by those skilled in the solid electrolyte fuelcell art that the critical. behavior of dissimilar materials, at hightemperatures, or abrupt changes intemperature, especially createsa gastight, high strength junction problem. In the present invention, thisproblem is minimizedrby a substantial reduction in size of individualfuel cell elements.

Reduction insize of individual fuel cell elements and the particularconfiguration thereof in the present invention enables a reduction inthickness of the solid electrolyte compatible with rigidity andmechanical strength requirements and consequent improvement in internalresistance characteristics: of each individual fuel cell element.Assembly of a greater multitude of fuel cell elements in parallelelectrical connection fur ther improves the overallinterrial resistancecharacteristic of the fuel cell unit with consequent lower ohmic losses.Lower ohmic losses means less internal heat is generated, of course, andthis in turn improves the op erating temperature characteristic andenables better temperature control over extended periods of use.

Moreover, it has been found that the present invention may bemanufactured in a fast, expedient manner utilizing straightforwardmanufacturing techniques of proven reliability in a prescribed order.The manufacturing method of this invention is relatively inexpensive toperform, provides a planar assembly which may be stacked in spacedrelation, by use of similar planar assemblies, and affords production oflow cost, high voltage, fuelcell units.

A more complete understanding of the device of this invention will behad upon a review of the detailed specificationand the drawings wherein:

FIG. 1 is a cross section view ofa single finite fuel cell element inaccordance with the invention.

FIG. 2 shows a top view of a typicalfuel cell element grouping in acommon plane embodiment of the invention.

FIG. 3 depicts a typical planar structure incorporating a battery offuel, cell element groupings as taught herein.

FIG. 4 is a schematic showing of a high voltage, high power, fuel cellin accordance with a stacked assembly embodiment of the invention.

FIG. 5 is a cross section showing of one stacked assembly embodiment.

FIG. 6 is a cutaway perspective showing of a high voltage, high power,fuel cell in accordance with a preferred stacked assembly embodiment ofthe invention.

FIGS. 7a, 7b and 7c depict typical masking items for use in themanufacture of the preferred embodiment of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The single finite fuel cellelement shown for illustrative purposes in.FIG. 1 comprises a thinsubstrate 11 which is an electrical insulator of glass, ceramic or othersimilar material. For reasons which will become apparent hereinafter,the thickness of the substrate should be substantially uniformthroughout its surface area witha minimum thickness compatible withmechanical strength requirements of the fuel cell planar structure. In atypical case, a ceramic substrate of 1mm thickness might be employed.

As shown in FIG. 1, the substrate is perforated and the perforationis'filledwitha solid electrolyte material l2which serves as themediumwhich functions to conduct mobile ions between its surfaces. A typicalmobile ion medium in this invention mightnbe phosphoric acid in teflon.Itwill be appreciated that the solid electrolyte 12 must. completelyfillthe perforation in the substrate l1 and that the Solid electrolytematerial must be impervious to gas such as hydrogen and oxygen as theperforated substrate serves as a physical barrier dividing the fuel cellinto separate fuel and oxident chambers.

An electrocatalyst material, indicated at 13 and 14, having a largersurface area than the cross sectional area of the solid electrolyte 12covers the two end face surfaces thereof and overlaps the substrate 11about the periphery of the perforation. It will be recognized thatplatinum or, for example, any Group VIII series metal of the periodictable may be employed as the electrocatalyst material in this invention.In the case of platinum, the thickness of the electrocatalyst coatingmight be 50 angstroms. It will be appreciated that this minimal preciousmetal requirement represents a sub stantial cost savings in solidelectrolyte fuel cell design.

In accordance with the invention, both sides of the entire substrate aresubstantially covered by a conduc tive coating indicated at and 16,except for the perforated regions thereof where the center portion ofthe electrocatalyst material indicated at 13 and 14 covering the endfaces of the electrolyte 12 is not coated and thus is exposed forreaction with its appropriate fuel or oxidant gas.

It will be recognized that the thickness of the conductive coating whichmay be silver, for example, is not critical provided, of course, itmeets electrical current carrying requirements of the surface. In atypical instance, the thickness of a' silver conductive coating might be50,000 angstroms.

FIG. 2 shows a top view of a parallel connected grouping of twenty-fourfuel cell elements, indicatedat 22, on a substrate 11 with oneelectrolyte free perforation indicated at 23. For reasons which willbecome apparent, the upper conductive surface 15 covers each of thetwenty-five perforations whereas the lower conductive surface 16includes a tab section, indicated at 24, and is cutaway, as indicated bythe dotted line at 25, in the vicinity of the electrolyte freeperforation 23.

It will be appreciated that twenty-five perforations have been shown forillustration purposes only and that any number of perforations pergrouping may be utilized in the parallel connection of fuel cellelements. Likewise, the rectangular configuration of the grouping is notcritical and may be modified as desired. For example, a squareconfiguration with 11 perforations per side (instead of4 per side asshown in FIG. 2) containing a total of 222 perforations including 221fuel cell elements and one unfilled perforation has been utilized toadvantage in a series connection of parallel grouping on a singlesubstrate as hereinafter described.

A battery of fuel cell element groupings 31 may be produced on a singlesubstrate surface 11 as indicated in the top view presentation of FIG. 3In this embodiment, each of the fuel cell element groupings may besubstantially as shown in FIG. 2 with a cutaway section, not shown inFIG. 3, in the lower conductive coating in the vicinity of at least oneelectrolyte free perforation and a lower conductive coating tab section.

In accordance with this invention, each of the fuel cell elementgroupings is connected in series with at least one adjacent fuel cellelement grouping. That is, the upper conductive coating of each groupingis electrically connected through the electrolyte free perforation tothe lower conductive coating of one adjacent fuel cell element grouping.This electrical connection may be a simple wire connection, as indicatedat 32, or

in a preferred embodiment, utilizing the lower conductive surface 16configuration shown in FIG. 2 may be accomplished by filling theelectrolyte free perforation with a current conductive material such assilver. In the preferred embodiment, each of the end groupings in theseries connection thereof is electrically connected to a conductivestrip in close proximity to one edge of the substrate 11. For example,the upper conductive surface of one end grouping is electricallyconnected to conductive strip 33, as shown at 34, and the lowerconductive surface of the other end grouping is electrically connectedin like manner to a similar conductive strip not shown in FIG. 3 on theopposite side of the substrate 11. It will be appreciated that theelectrical connection of the end groupings to respective conductivestrips may be a simple wire connection as shown in FIG. 3 or in apreferred embodiment may be accomplished by a plated conductive stripand a plated electrical connection. While it is not essential to theinvention that the conductive strips on each side of the substrate 11 bein close proximity to the same edge of the substrate, it will berecognized that such disposition facilitates stacking of like units inthe manner shown in FIG. 5 with simplified electrical interconnection ofstacked units.

FIG. 4 shows in schematic form a relatively high voltage embodiment ofthe device of this invention wherein a plurality of the substrateassemblages shown in FIG. 3 are stacked in a substantially parallelspaced arrangement with the upper and lower surfaces of each substrateassemblage supplied with respective fuel gas and oxidant gas flowing inthe direction shown. As indicated in FIG. 4, one side surface may besupplied with a fuel gas such as hydrogen and another side surface maybe supplied with an oxidant gas such as oxygen such that the hydrogenand oxygen gases flow in directions orthogonal to each other and to thedirection of current flow in the high voltage embodiment, which isindicated by the arrow marked I. It will be appreciated that theorthogonal gas flow arrangement permits a relatively simple, compact,low cost manifold plenum design.

FIG. 5 is a cross section showing of a series connected stackedassemblage in accordance with the schematic showing of FIG. 4 depictingfour planar batteries of fuel cell groupings each on its own substratewith both surfaces of each battery individually subjected to itsrespective fuel or oxidant gas flow. More particularly, four substrates,indicated at 41, 42, 43, and 44, are shown with three gas imperviousmetallic separators, indicated at 45, 4 6'and47, interspersed thesubstrate and with two gas impervious end plates, indicated at 48 and49. Each of the substrates 41 through 44 are separated from respectiveend plate separators by metallic spacer members, indicated at 51 and 52.To accomodate the orthogonal direction of gas flow, the spacer members51 may be hollowed or slotted, as shown, and the spacer member 52 may besolid, as shown, to form eight gas flow chambers, indicated at 53through 60. In this embodiment, the metallic spacer members 51 and 52are disposed in allignment and electrical contact with the edge disposedconductive strip or respective sides of each substrate. Thus, the outputof the stacked assemblage may be taken across the terminals 61 and 62with the voltage determined by the product of (the number of groupingsper substrate) times (the number of substrates) times (1.1 volts) andthe current determined by the current capacity of each element grouping.It will be appreciated that this embodiment provides a relatively highvoltage output but that the power density of the assemblage iscontrolled by the current capacity of each grouping of fuel cellelements.

FIG. 6 is a cutaway perspective showing of a preferred, more compact,stacked assemblage in accordance with the invention wherein theseparators shown in the embodiment of FIG. 5 are omitted, the number ofgas flowchambers is reduced and consequently the height of the stackedassemblage is reduced. More particularly, four substrates exemplar-ilyindicated at 41, 42, 43 and 44 are shown with two gasimpervious endplates 48 and 49. In this embodiment however, the spacers indicated at71 and 72 are of an electrical insulator material. As in the embodimentof FIG. 5, to accomodate the orthogonal direction of gas flow, thespacer members 71 may be hollowed or slotted, as shown, and the spacermembers 72 may be solid, as shown, thus forming five gas flow chambers73, 74, 75, 76 and 77. Manifold and associated plenum chambers 81 and 82attached to hydrogen gas supply means, not shown, service the gas flowchambers 74 and 76. Likewise, manifold and associated plenum chambers 83and 84 attached to oxygen gas supply means, not shown, service the gasflow chambers 73, 75 and 77.

In accordancewith a preferred embodiment of the invention, the stackedassemblage of FIG. 6 is electrically connected in parallel by conductors85 and the output of the stacked assemblage may be taken across theterminals 91 and 92. It will be seen that the output voltage of theembodiment of FIG. 6 is determined by the product of (the number ofgroupings per substrate) times (1.1 volts). The current capacity, on theother hand, is determined by the product of (the current capacity ofeach grouping of fuel cell elements) times (the number of substrates).Thus, the embodiment of FIG. 6 affords a relatively high density, highvoltage power source.

Moreover, it will be recognized that each of the substrate members hassubstantially the same physical configuration and may be manufacturedutilizing mass production techniques with consequent cost and qualityassurance advantages.

It will beappreciated that the basic planar structure of the fuel cellof this invention, shown in FIG. 3, may be manufactured in largequantities at relatively low cost by the method to be described herein.For purposes of explanation, the method of manufacture is directed tofuelcell element groupings such as shown in H65. 2 and 3.

In accordance with the method of manufacture, as a first step, arectangular thin glass or ceramic substrate is perforated during thecourse of manufacture of the substrate by precise placement of aplurality of fine rods laid out in a specified grouping pattern and therod material, which extends through the substrate, is subsequentlyremoved by dissolving, melting, mechanical drilling or otherwise asappropriate. Alternatively, the first step of perforation of thesubstrate may be accomplished after manufacture of the substrate byconventional diamond dust fine hole drilling techniques. Obviously, thealternate method of perforation is more time consuming and expensivethan the first described method and therefore, the first describedmethod of obtaining perforations is preferred in mass productionefforts.

As a second step, the two sides of the substrate are coated with arelatively thick film (50,000 A) of electrically conductive materialsuch as silver or gold utilizing the masks shown in FIGS. 7a, and 7b foreach grouping. In a typical-case, the conductive material might bedeposited in a conventional sputtering fixture. As the perforation inthe substrate will be filled with electroltye in a subsequent step, theconductive coating of step 2 must not close these perforations. It hasbeen found that by use of the sputtering technique, the finiteperforations will remain open.

As the conductive material pattern of each element grouping does notdiffer essentially on each respective surface of the substrate, the maskshown in FIG. would be used to provide the conductive coating on onesurface of the substrate and the mask shown in FIG. 7b would be used toprovide the conductive coating on the opposite surface of the substrate.

As a third step, the perforated and coated substrate is filled with animmobilized (solid) electrolyte such as phosphoric acid in Teflon. Thisstep may be accomplished by forcing a finely ground Teflon powder intothe perforations by conventional rolling or plate and press pressuretechniques which serve to compact the Teflon powder into a bonded stateand subsequently soaking the filled substrate in a bath of phosphoricacid for a period of time sufficient to impregnate the bonded Teflonpowder. It will be recognized that the mobile ion medium, in thisexample, phosphoric acid impregnated Teflon powder, may be constitutedprior to filling the perforation in the substrate, if desired. However,it has been found that unadulterated Teflon powder is readily workedinto the perforations and compacted therein and that subsequenttreatment of the Teflon powder is preferred. As the next stepnecessitates relatively smooth substrate surfaces, the filled substrateis cleaned and leveled by mechanical scraping, or other wise, inpreparation therefor.

The fourth'step involves sputtering a catalyst mate rial, such asplatinum over both exposed surfaces of the electrolyte using thecatalyst mask shown in FIG. 7c. As the diameter of the holes in the maskis slightly larger than the diameter of the perforations, the catalystmaterial serves to seal the mobile ion medium, the solid electrolytewithin the coated substrate.

Next, as a fifth step, one corner disposed filled perforation of eachgrouping is drilled or punched to remove the solid electrolyte containedtherein. As this electrolyte free perforation is to be used :as anelectrical interconnection between surfaces, a tinned wire may bethreaded through the electrolyte free perforation if desired. However,as a practical matter, it has been found that by sufficient enlargementof the perforation, the threaded wire may be eliminated.

It will be recognized, especially in consideration of the finite size ofthe fuel cell elements, that precise placement of the masks is essentialto the manufacture of the planar structure and that the register notchesshown on two edges of each of the masks enable such precise placement.Moreover, it will be appreciated that in actual manufacture of theplanar structure all element groupings on a single substrate would bemanufactured simultaneously utilizing a complex multigrouping mask, notshown, at each coating stage of the manufacturing process.

The method of making terminal connections on each surface of the planarstructure is not critical and any printed circuit technique, or thelike, may be employed to enable serial interconnection of planarstructures in accordance with the embodiments of FIGS. or 6. Forexample, a lead wire may be soldered to the appropriate groupingconductive coating on each surface for external interconnection asprescribed herein.

In making the high voltage fuel cell embodiment of FIG. 6, the planarstructures are stacked in spaced relation and the end plate and gasplenums attached to the stacked assembly in a conventional manner asdictated by straightforward mechanical considerations.

Following the teaching of this disclosure, a fuel cell stacked assemblyhaving a 62.2 ampere, 157.5 volt, 10,000 watt rating may be built in arelatively small cube unit approximately l3 inch by 13 inch per side. Insuch a stacked assembly, 222 planar structures would be incorporatedwith 121 element groupings in each planar structure and with 221 fuelcell elements per grouping. In such a planar structure each fuel cellelement would be contained in a 1.0 mm diameter perforation with a 0.3211 mm spacing between perforations with each fuel cell element having aneffective cell area of 0.78540 sq. mm. In accordance with calculations,each fuel cell element would have an internal impedance of [9.48 ohms,and would generate 0.000888 watts with a heat energy loss of 0.000031watts.

While the fuel cell assembly of this invention and especially the planarstructure thereof have been described with particular reference tospecific embodiments, it is understood that this invention is notrestricted to the exemplarily shown embodiments. Likewise, the method ofmanufacture of the planar structure may be other than as specificallyset forth without departing from the general purview of the disclosure.

1 claim:

1. The method of producing the basic planar structure of a stacked solidelectrolyte fuel cell which comprises the steps of:

a. perforation of a thin substrate member in a precise pattern to obtaina predetermined multitude of finite holes therein extending through saidthin substrate member from surface to surface; then b. masking saidsurfaces of said holes and sputter depositing a like plurality ofelectrically conductive coatings on the unmasked areas of said surfacessuch that pluralities of perforations are grouped between pairs ofcoatings on opposite sides of the thin substrate; then filling theperforations with a solid electrolyte ion mobility medium and levelingthe filled perforations to obtain smooth surfaces on opposite faces ofthe thin substrate; then d. masking said previously unmasked surfacesand electrically connecting the conductive material of each fuel cellelement grouping on one surface of the substrate to the conductivematerial of at least one adjacent fuel cell element grouping on theopposite surface of the substrate such that a serial electricalconnection of fuel cell element groupings is obtained;

wherein as a part of step (b) said electrically conductive coatings areselectively cutaway and tabbed such that said tabs sre disposed inparallel relation to and in close proximity to a portion of a conductivecoating of an adjacent fuel cell grouping, the last said portion of aconductive surface being deposited on the opposite surface of saidsubstrate;

and wherein the electrical connection described in step (e) is madebetween each tab and said portion of the conductive coating on theopposite surface

